Hi Miranda,
If you are planning to spend one full day at the Magic Kingdom and one full day at Epcot, a 2-Day Ticket with Admission to 1 Park Per Day would be your best bet.
Tickets and any options purchased must be used within 14 days of first use.
Seasonal pricing is now in effect for 1-Day Theme Park Tickets. The "
when can I use my 1-day theme park ticket?" question and answer on the Tickets - Frequently Asked Question section of the Walt Disney World website explains this.
1-Day tickets purchased after February 28, 2016 must be used by December 31, 2017, but the amount paid for an unused, expired ticket may be applied towards the purchase of a new ticket at the current price so long as the new ticket purchase price is equal to or greater than the amount paid for the original ticket.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma